Nowadays, fluorescent materials are widely used as illumination or display materials, and support our daily lives. Among such fluorescent materials, inorganic matrices with coloring materials and/or transition element ions (transition-metal ions and/or rare earth ions) have been in use for a long time.
In recent years, it has been discovered that semiconductor nanoparticles obtained by a thoroughly researched manufacturing method emit light efficiently. Among such semiconductor nanoparticles, II-VI group compounds, such as cadmium telluride, and the like, are typically mentioned, and have a diameter of approximately 2 to 5 nanometers. Because semiconductor nanoparticles have a short emission lifetime, and because the emission wavelengths can be controlled by changing particle diameters, such semiconductor nanoparticles are attracting attention as a new luminescent material.
Such semiconductor nanoparticles have a large specific surface area because of their small particle sizes. Thus, in order to suppress radiationless deactivation, and moreover, to raise the fluorescence quantum yield while suppressing aggregation of the particles, a deactivation treatment of reducing the number of surface defects by a surface treatment is usually performed. For such a surface treatment, organic surfactants containing sulfur, such as thiols and the like, and/or zinc sulfide are preferable.
As a method for producing such semiconductor nanoparticles, using a surfactant in an aqueous solution is well known (Non-Patent Document 1). However, the semiconductor nanoparticles produced by the aqueous solution method are unstable while still in the aqueous solution state, and are not suitable for industrial applications.
There is, for example, a report concerning a method for fixing such semiconductor nanoparticles in an organic polymer (Non-Patent Document 2). However, polymers used as a matrix have low levels of light, heat, and chemical resistance, as well as other properties, and gradually permit the passage of water and oxygen. The resulting drawback is a gradual degradation of the fixed nanoparticles.
To overcome the drawbacks of such polymer matrices, there are reports concerning methods for dispersing semiconductor nanoparticles in a glass matrix by a sol-gel process. For example, Patent Document 1 discloses that a sol gel method using organoalkoxysilane can provide a fluorescent material in which semiconductor nanoparticles with a fluorescence quantum yield of 3% or more are dispersed in a solid matrix containing silicon at a concentration of 5×10−4 to 1×10−2 mol/l. Patent Document 2 discloses a fluorescent material in which semiconductor nanoparticles with a fluorescence quantum yield of 20% or more are dispersed in a matrix formed by a sol gel method at a concentration of 2×10−6 to 2×10−4 mol/l.
These fluorescent materials are obtained by a sol gel reaction by mixing semiconductor nanoparticles, a surfactant, organoalkoxysilane, and the like, and have semiconductor nanoparticles dispersed in a glass matrix. Therefore, the materials can prevent degradation of the semiconductor nanoparticles, and thereby have excellent improved stability over time.
However, a fluorescent material produced by the simple mixing mentioned above does not always reach the required high brightness level. In order to obtain a light-emitting device with a higher brightness, semiconductor nanoparticles with a higher fluorescence quantum yield need to be uniformly dispersed in a matrix at higher concentrations without aggregation. In this regard, there is room for further improvement in the above-described fluorescent materials of Patent Documents 1 and 2.
In recent years, there has been a report concerning a method for forming a thin film comprising semiconductor nanoparticles with high packing density using a layer-by-layer method utilizing chemical adsorption. In the layer-by-layer method, a surface-treated substrate is alternately immersed in at least two kinds of solutions for a given period of time, thereby laminating semiconductor nanoparticles and matrices individually on the surface. In most cases, since coating is performed for every thickness close to that of a monomolecular layer, a thin film with a high packing density of nanoparticles is formed.
As examples of methods for forming thin films comprising semiconductor nanoparticles using this method, Non-Patent Documents 3 to 8 can be mentioned.
Non-Patent Document 3 discloses a fluorescent material in which cadmium telluride nanoparticles (a littler less than 3 nanometers in diameter) are dispersed in matrices comprising a carbazole copolymer and polyacrylamide. The concentration of nanoparticles is 0.05 mol/l. However, the wavelength of fluorescence is sharply shifted to blue and shifted from the visible region, and it is observed that the matrix emits light.
Non-Patent Document 4 discloses a fluorescent material in which cadmium sulfide nanoparticles (3 to 4 nanometers in diameter) are dispersed in a matrix comprising polydiallyldimethylammoniumchloride. The concentration of nanoparticles is 0.003 mol/l, and light emitted from the fluorescent material exhibits an extremely wide spectral bandwidth because the light is emitted from a defect.
Non-Patent Document 5 discloses a fluorescent material in which cadmium telluride nanoparticles (4 nanometers in diameter) are dispersed in a matrix comprising polydiallyldimethylammoniumchloride. The concentration of nanoparticles is 0.01 mol/l. Although the fluorescence quantum yield in a solution state is 20%, the fluorescence quantum yield in a matrix is sharply reduced to as low as 5%.
Non-Patent Document 6 discloses a fluorescent material in which only a single layer of cadmium selenide nanoparticles is adhered to polydiallyldimethylammoniumchloride, and the like, as a base. The fluorescence quantum yield of the nanoparticles before adhesion is as low as 4.2%, and after adhesion, it is estimated to be lower than 4.2%.
Non-Patent Document 7 discloses a fluorescent material in which cadmium sulfide nanoparticles (about 6 nanometers in diameter) are dispersed in a matrix comprising a long-chain alkyl thiol at a concentration of 0.001M. There is no data on light being emitted from the fluorescent material.
However, in the known literature described above, an organic matrix is used, and the wavelength of fluorescence is sharply shifted to blue after nanoparticles are put into the matrix compared to the previous state, and moreover, the fluorescence has a considerable tail towards red due to defects on the surface of the nanoparticles in many cases. In any of the cases above, the fluorescence quantum yield can be estimated to be at most a few percent. In addition, an organic matrix is adopted, which causes problems such as low light, heat, and chemical resistance, moisture permeability, gas permeability and the like. Moreover, the fixed semiconductor nanoparticles deteriorate gradually, causing poor long-term stability.
Such display materials are faced with continuous demands for higher brightness and resolution driven by the current digital boom, and a method for reaching the maximum brightness is desired. Therefore, expectations are high for a thin-film fluorescent material having a high level of brightness in which semiconductor nanoparticles are held at high concentrations in a matrix while maintaining a high fluorescence quantum yield. However, under the present circumstances, a satisfactory thin-film fluorescent material cannot be provided.    Patent Document 1: WO 2004-000971, pamphlet    Patent Document 2: WO 2004-065296, pamphlet    Non-Patent Document 1: Gao, et al., Journal of Physical Chemistry, B, vol. 102, p. 8360 (1998)    Non-Patent Document 2: Bawendi, et al., Advanced Materials, vol. 12, p. 1103 (2000)    Non-Patent Document 3: Yang, et al., Journal of Materials Chemistry, vol. 13, p. 1356 (2003)    Non-Patent Document 4: Halaoui, Langmuir, vol. 17, p. 7130 (2001)    Non-Patent Document 5: Kirsten et al., Materials Science and Engineering C, vol. 8 to 9, p. 159 (1999)    Non-Patent Document 6: Kotov et al., Langmuir, vol. 18, p. 7035 (2002)    Non-Patent Document 7: Akamatsu et al., Langmuir, vol. 20, p. 11169 (2004)